In bug 567931 we found that virt-top would exit occasionally
when the terminal window was resized. Tracking this down it
turned out that SIGWINCH was being delivered to the process at
exactly the point where the libvirt remote driver was calling
poll(2) waiting for a reply from libvirtd.

This caused the poll(2) call to be interrupted (returning errno
EINTR). However handling EINTR the same way as EAGAIN was not
the solution to this problem since we found previously that this
would break Ctrl-C handling (commit 47fec8eac2bb3).

The correct solution is to mask out SIGWINCH for the duration
of the poll(2) system call. The per-thread mask is changed and
restored immediately after the call. Since we are using
pthread_sigmask, this should not affect other threads, and
since we restore the signal mask immediately afterwards it should
not affect the current thread visibly either. Other possibly
problematic signals are SIGCHLD and SIGPIPE and these are
masked too.

Note use of ignore_value: It's not fatal if we cannot mask out
SIGWINCH, and in any case pthread_sigmask never fails on Linux
as long as you supply the correct arguments.